1. Field of the Invention
The present invention relates to a connector for a printed circuit board and, more particularly, to a terminal guide of lead terminals used in a right-angle type connector for a printed circuit board.
2. Description of the Related Arts
There are two types of connectors for printed circuit boards: straight-type connectors in which lead terminals are led straight in order to mount the connector perpendicularly to the printed circuit board and right-angle type connector in which lead terminals are bent at a right angle to enable the connector to be mounted in parallel with the printed circuit board.
A right-angle type connector for printed circuit boards has a problem in that, when the number of the lead terminals is large, it is difficult to maintain the pitch or spacing of the lead terminals in conformity with the pitch of the holes which are formed in the printed circuit board to receive the lead terminals. This problem causes an impediment to the use of an automatic assembly system which is becoming popular in recent years. Whether an automatic assembly system can be adopted significantly depends on whether the lead terminals can possibly be arranged at such a small pitch is possible.
Japanese Patent Laid-Open No. 60-98280 shows an example of such a known right-angle type connector for printed circuit boards. The connector has a housing which is provided at both ends thereof with protruding tabs and notches having groove are formed in the housing at the inner side of the protruding tabs. Orthogonally-bent terminals are mounted on the housing. A terminal guide is provided that has lead terminal insertion holes formed at a pitch corresponding to the pitch of the lead terminals. Projections for engagement with the above-mentioned grooves in the housing are formed on both ends of the terminal guide.
In use, after the lead terminals are mounted on the housing, terminals are inserted into the lead terminal insertion holes in the terminal guide. Then, both ends of the terminal guides are put into engagement with the notches near the tabs and the projections are resiliently engaged with the grooves, thus holding the leads correctly at a predetermined pitch.
The following problem arises when the lead terminals are mounted by soldering their ends to the printed circuit board. For instance, a glass epoxy resin having a thermal expansion coefficient of 0.1% is used as the material of the printed circuit board while the material of the terminal guide is,for example, a reinforced polyethylene terephthalate having a thermal expansion coefficient of 0.3%, Consequently, if heat is applied after independent soldering, stress is concentrated to the ends of the lead terminals and the root portions of the solder, due to difference in the thermal expansion, i.e., due to difference in the thermal expansion coefficient between the printed circuit board and the terminal guide. Consequently, cracks or separations appear at the soldered portion, resulting in an inferior contact.
In another assembly method, ends of the lead terminals are automatically soldered to the printed circuit board and then the assembly is cooled down to the normal temperature. Stress concentration also takes place in this method in the regions between the ends of the lead terminals and the soldered portions of the printed circuit board, due to a difference in the amount of thermal contraction caused by the difference in the material. Cracks or separations also take place in this case, with the result that the reliability of electrical connection is impaired.